In the polymerization of alpha-olefins, it is known to use catalyst systems comprising a transition metal compound and an organometallic compound. It is further known that the productivity of such catalysts can generally be improved if the transition metal compound is employed in conjunction with a metal halide such as magnesium halide. U.S. Pat. No. 3,642,746 discloses that particularly active catalysts can be prepared by pretreating solid particles of a dihalide of a divalent metal such as MgCl.sub.2 with an electron donor and then heating the thus pretreated solid with a transition metal compound such as TiCl.sub.4.
An object of the present invention is to provide a method for preparing more highly active olefin polymerization catalysts.
Another object is to provide a new process for polymerizing olefins using such catalyst.